Plasmacytoid tumours in mice and Burkitt's lymphomas in humans display characteristic chromosomal translocations involving the c-myc proto-oncogene. Models to explain quantitative changes in c-myc expression have been proposed based on the loss of normal promoters as a result of translocation. However, alternative explanations, such as somatic mutation are needed to explain altered c-myc expression in the absence of gene breakage. We present here the nucleotide sequence of the normal murine c-myc gene. Comparison of this sequence with that of a translocated c-myc gene from a murine plasmacytoma reveals complete identity of coding sequence. One nucleotide difference was found in the non-coding first exon. This shows that qualitative changes of the c-myc gene product are not required for oncogenesis in murine plasmacytomas. In contrast, mutations are found in coding and non-coding regions of translocated c-myc genes from Burkitt's lymphomas, suggesting that the mechanisms by which c-myc is activated in plasmacytomas and Burkitt's lymphomas are different.